In recent years, there has been a rapid increase in demand for delivery of real-time applications and services in computer networks, including Pseudo-Wire Emulation (PWE). Voice over IP (VoIP), video conferencing, and broadcast, multicast and manycast streaming services such as H.261, H.323, and IPTV. These real-time services typically require highly accurate timing to ensure high service quality. Highly accurate timing is also required at base stations in wireless networks based on technologies such as Global System for Mobile communications (GSM), Code Division Multiple Access (CDMA), WiMAX, and Long Term Evolution (LTE).
To ensure high service quality and to facilitate network management, it is desirable to eliminate clock mismatch between computer network equipment such as switches, routers, and base stations. This can be done by providing a highly accurate timing reference at each node, such as a Global Positioning System (GPS) reference or a lower quality oscillator such as a Stratum 2 rubidium oscillator, where the specification for Stratum 2 clock quality is given in Telcordia GR-1244-CORE. However, at the same time it is desirable to reduce the substantial cost resulting from per-node deployment of these timing references.
To reduce per-node cost, it is desirable to use a cheaper oscillator, such as an oven controlled crystal oscillator (OCXO) or a temperature controlled oscillator (TCXO), at each client node. However, OCXO's and TCXO's may be unable to meet Stratum 2 clock quality requirements, or the corresponding clock quality requirements for wireless networks defined in ITU-T G.823 SEC, 3GPP, and IEEE 802.16e. To solve this problem, one or more clock servers may provide timing through a data network to each client node. Each clock server may obtain timing derived from a Stratum 1 reference, such as the Global Positioning System (GPS) or a primary reference source locked to GPS. Each client may recover frequency and absolute phase information from this reference clock source.
One of the important factors that limits the accuracy of timing distribution in packet networks is variations in network delay, known as jitter or packet delay variation (PDV) over time, experienced by timing packets sent between a clock server and client nodes. Jitter is typically considered to include short-term variations in the packet delay. A quality of service (QoS) policy that is frequently applied to timing packets is Expedited Forwarding (EF). Among EF traffic and any other lower priority traffic, the EF traffic is queued and transmitted first. Even so, timing packets still can experience jitter resulting from the multiplexing of timing packets with other timing packets or with lower priority data packets. Many packet based systems use store-and-forward media access techniques. These systems typically receive an entire packet and queue the packet to an egress media access port. Before the packet can be transmitted on the egress port, any previous packet that is already in the process of being transmitted should be fully transmitted. This process of queuing before egress can cause a large jitter for any particular packet from any particular client device.
When multiple clients transmit timing packets at regular intervals based on recovered absolute phase information, many of these timing packets tend to arrive at the network edge at approximately the same time, especially in networks in which the multiple clients are in relatively close proximity. These timing packets tend to accumulate in a burst of many packets that can then be forwarded to the server (or a set of servers). FIG. 1 illustrates an example of timing packets arriving at a server from multiple clients, in accordance with prior art. Timing packets 100 arriving at the server from each of eight clients are marked as “a” from the first client, “b” from the second client, and so on to “h” from the eighth client. One timing packet 100 is transmitted from each client during each time interval T, so that one timing packet 100 from each client arrives as part of a burst 101 of timing packets that arrives at the server during each time interval T. The bursts 101 arrive at the server starting at each of arrival times tarr, tarr+T, tarr+2T, and so on. As illustrated in FIG. 1, during any given time interval T, the timing packets 100 transmitted by a given client may arrive first, last, or elsewhere in the burst. The order of the timing packets 100 in each burst depends on many factors such as the accuracy of the phase estimation at each client, the recovered frequency at each client, and queuing effects within the network traversed by the timing packets.
The recovered frequency and phase at each client typically includes jitter and wander. Wander is typically considered to include long term variations in the packet delay, such as on the order of one-tenth of a second or greater. This jitter and wander in the recovered frequency and phase can increase as a result of the increased jitter experienced by timing packets as a result of the bursting and variation in ordering of timing packets illustrated in FIG. 1. In addition, increased wander in the recovered frequency and phase can lead to slowly varying variations in the ordering of timing packets, which can lead to further increases in the wander of the recovered frequency and phase. The magnitude of the wander can be on the order of milliseconds, which far exceeds the absolute phase accuracy requirement of about 3 microseconds for wireless protocols such as CDMA, TD-SCDMA and LTE-FDD. This base station to base station phase accuracy requirement is needed to maintain call quality, reduce interference, and avoid dropped calls.
It is therefore desirable to determine how to reduce the likelihood and length of bursts of timing packets in telecommunication networks, especially in networks including multiple clients that transmit timing packets at regular intervals based on recovered absolute phase information.